The invention pertains to electrical switches and more particularly to such switches utilizing capacitive coupling.
Electrical switches can be used for many control and selection functions. One such widely used function is in keyboard devices such as typewriters, telephone handsets and calculators. The keyboards of such devices, since they control electronic signals, try to avoid the use of mechanical contact switches because of their complexity, unreliability of bounce and cost. Thus, these keyboards utilize a variety of transducers to perform the switching operation. People have tried to use magnetically operated reed switches, Hall effect devices, photo-electronic devices and capacitive techniques. In order to improve the reliability of the operation of the keyboard, each key switch has a simple compression spring to provide a key switch restoring force. Some recent keyboard designs eliminate the use of compression springs by relying on magnetic restoring forces effected by the interactions of fixed and movable magnets. Some of these magnetic techniques simply and reliably simulate the feel of the special and complicated "over-center" and/or other special "tripping" mechanisms of some keyboards.
However, all of these electronic keyboards have less mechanical hysteresis than conventional "over-center" key switches.
Mechanical hysteresis is the phenomenom which occurs when a key is depressed and an electrical signal is generaged at a first point during the downward travel of the key. This signal cannot be generated again until the key is released and travels a certain distance to a second point above the first point and then travels downward again past the first point. The distance between the first and second points is the mechanical hysteresis. However, it is usually expressed as percentage of total key travel. For example, if the spacing between the first and second points is 0.010 inch and the total possible key travel is 0.20 inch then the key is said to have 5% mechanical hysteresis.
It has been found that capacitive switch devices wherein the key acts as a coupling element between two coplanar pads have one of the smallest mechanical hysteresis phenomena. The reason is that practically all of the useful signal coupling occurs during the last 5 to 15 percent of the total travel of the key and during this range the coupling changes sharply. What is desired is that the useful signal coupling occur over a much broader range and that the change be less sharp. In this way one can sensitize the electrical reading circuits to discriminate between two levels of signal coupling at two more distant points in the travel.
In spite of this poor mechanical hysteresis property of conventional "capacitive keyboards," they afford the greatest economy of manufacture since they rely on the positions of conductive elements that are easily printed on substrates. Therefore, it is highly desirable to utilize a capacitive keyboard.